[notmuch] (no subject)

[david at tethera.net]

Thanks to Micah Anderson on IRC for pointing out that the previous
version of this patch broke '?' in notmuch-show mode.  This was
arguably a bug in the notmuch help code, fixed in patch 2 of the
series.  I wouldn't be surprised if 'map-keymap' could be used other
places in the code, but I kept things minimal-ish.  The first patch 
has been revised to actually include documentation strings (blush).